AbstractPien Tze Huang (PZH), a well-known traditional Chinese formula prescribed 450 years ago in the Ming Dynasty, has been used in China and Southeast Asia for centuries as a folk remedy for various types of cancer, including colorectal cancer (CRC). Recently, we reported that PZH is capable of inhibiting colon cancer growth both in vivo and in vitro via the promotion of apoptosis and inhibition of tumor angiogenesis. To elucidate the mechanism of the tumoricidal activity of PZH, its effect on the proliferation of human colon carcinoma Caco-2 cells was evaluated and the underlying molecular mechanism was investigated. Results showed that PZH inhibited Caco-2 cell viability and survival in a dose- and/or time-dependent manner. In addition, PZH treatment was found to block the G1/S cell cycle progression. Moreover, PZH suppressed the mRNA and protein expression of pro-proliferative Cyclin D1 and CDK4. Findings of the present study suggest that inhibition of cell proliferation via the G1/S cell cycle arrest is a potential mechanism by which PZH can be effective in the treatment of cancer.